In recent years, because of enhanced awareness of alternatives to petroleum resources and environmental consciousness, there has been a focus on materials utilizing reproducible natural fibers. Among natural fibers, cellulose fibers having a fiber diameter of 10 μm or more and 50 μm or less, in particular, wood-derived cellulose fibers (pulp) have been widely used, mainly as paper products, so far.
In addition, ultrafine fibrous cellulose, which have a fiber diameter of 1 μm or less, is known as cellulose fibers. Such ultrafine fibrous cellulose can be used as a raw material for constituting a sheet or a complex. It has been known that when the ultrafine fibrous cellulose is used, the contacts of fibers are significantly increased, and thus that the tensile strength of such a sheet or a complex is significantly improved. Moreover, the use of the ultrafine fibrous cellulose in a thickener or the like has also been considered.
Ultrafine fibrous cellulose can be produced by subjecting conventional cellulose fibers to a mechanical treatment. Cellulose fibers strongly bind to one another via a hydrogen bond. Accordingly, when ultrafine fibrous cellulose is produced by simply performing a mechanical treatment on such cellulose fibers, enormous energy is required to obtain the ultrafine fibrous cellulose.
It has been known that, in order to produce ultrafine fibrous cellulose with smaller mechanical treatment energy, it is effective to perform a pre-treatment such as a chemical treatment or a biological treatment, as well as the mechanical treatment. In particular, when a hydrophilic functional group (e.g., a carboxyl group, a cation group, a phosphoric acid group, etc.) is introduced into a hydroxyl group on the surface of cellulose by a chemical treatment, electrical repulsion is generated between ions, and also the ions are hydrated, so that the dispersibility, particularly, in an aqueous solvent is significantly improved. Hence, when compared with the case of not performing a chemical treatment, energy efficiency of fibrillation is increased.
For example, Patent Documents 1 and 2 disclose phosphorylated ultrafine fibrous cellulose, in which a phosphoric acid group forms an ester with a hydroxyl group of cellulose, and a method for producing such phosphorylated ultrafine fibrous cellulose. Patent Document 2 proposes that a phosphoric acid group introduction step is carried out in the presence of urea, and that the amount of the phosphoric acid groups introduced is increased by carrying out the phosphoric acid group introduction step multiple times.